In the therapy of several diseases, Transdermal Therapeutic Systems (TTS) have been introduced on the market for some time now.
Also, TTSs comprising the active substance estradiol have been on the market as therapeutic agents for climacteric complaints, and, since a short time ago, also against osteoporosis; they have proved successful in therapy.
In the following the term "estradiol" is to be understood as the anhydrous substance of 17-.beta.-estradiol.
A disadvantage of prior art systems is the insufficient capability of the active substance to permeate through the skin. This cannot be increased beyond a certain limit, the so-called "saturation flow", although several galenic measures with respect to the TTS-design have been taken (use of multilayer systems, use of controlling membranes, variation of the active substance concentration, modification of the base polymer, and the like). This finding that the transdermal flow of an active substance from the solid, finely dispersed phase cannot be increased further in principle, can already be found in the still trailblazing works of Higuchi (e.g., T. Higuchi; Physical Chemical Analysis of percutaneous absorption process from creams and ointments. J. Soc. Cosmetic Chem. 11, p. 85-97 (1990).
The systems described in EP 0 421 454 comprise estradiol in an acrylic polymer under addition of "crystallization inhibitors" and tackifying resins. Swelling agents are contained to give protection against premature loss of adhesive force.
In addition, with a lot of active substances, so-called "enhancers" can be added to the TTS during production. These are usually liquid admixtures improving the absorption properties of human skin; for this reason, they allow the absorption of the active substance from a sufficiently small TTS-surface.
Readily volatile enhancers, e.g., ethanol used for the active substance estradiol, particularly involve problems caused by an extreme softening of the TTSs' adhesive layers, and they require additional bulky compartments in the system, rendering the TTS unacceptably thick.
The addition of less volatile, however, mostly less active enhancers (e.g., glycerol esters, cyclic amides, eucalyptol) allows the production of matrix systems comprising active substances and an absorption-promoting component in one or several layers. However, the insufficient adhesive force of these TTSs is disadvantageous. U.S. Pat. No. 4 863 738 represents one of many examples claiming the application of active substances, e.g., estradiol, together with a certain enhancer (in this case glycerol monooleate) in an optional TTS-matrix and in an optional concentration.
However, even such a prior art TTS does not permit a satisfactory therapy either. The reason is that either the chosen enhancers are poorly tolerated by the skin or that the systems have unacceptably large surfaces owing to the still insufficient flow through the skin.
Another (theoretic) possibility of increasing the active substance flow through the skin is to dissolve more active substance molecularly disperse, i.e., crystal-free, in the TTS than corresponds to the saturation solubility. The permeation rate through the skin increases to the same extent as the degree of supersaturation of these systems. However, these physical states are thermodynamically unstable, therefore these forms of administration are not storable. Within some months or years, at the latest, a spontaneous unforeseeable precipitation of active substance will take place so that the flow rate through the skin gradually decreases to the saturation flow level; depending on the starting concentration, this results in losing a great portion of the initial therapeutic activity.
This process occurring during storage is due to particular physicochemical characteristic features of estradiol.
At room temperature and normal relative air humidity (20-60% relative humidity), estradiol is not present in one of the two known anhydric modifications (I and II) but as a semihydrate (Busetta, Acta Cryst. 1972, B28, 560). Owing to the layered structure stabilized via hydrogen bridges, and because of the diffusional compactness of the crystal compound, the hydrate can be subjected to a short-term heat treatment to temperatures of about 170.degree. C. without decomposition thereof (Kuhnert-Brandstatter and Winkler (1976) Scientia Pharmaceutica 44 (3), 177-190). However, estradiol-semihydrate can quantitatively be converted into the anhydrous form already at about 120.degree. C. by way of enlarging the crystal surface by means of micronizing. According to the inventors' observations, the transformation already takes place at about 90.degree. C. if heating is conducted slowly (0.2-1 K/min) and in case of a particularly fine substance.
With decreasing partial water vapor pressure, on the other hand, estradiol has a higher solubility in some polymers, particularly in polyacrylates. According to Fick's law, higher concentrations with otherwise same conditions increase the diffusion flow through the skin; for this reason such a concentration increase is very desirable in transdermal therapeutic systems. However, the water introduced with the estradiol-semihydrate is already sufficient to cause gradual recrystallization from the solution as estradiol-semihydrate (Kuhnert-Brandstatter and Winkler (1976) Scientia Pharmaceutica 44 (3), 177-190). During crystallization, the flow rate from the system to the skin considerably decreases with the diminishing concentration.
Accordingly, transdermal therapeutic systems are known that offer a pharmacotherapeutically satisfying solution by exactly regulating the concentration to below the saturation solubility of the estradiol-anhydrate (DE-PS 42 37 453) or by using partially undissolved, disperse estradiol-anhydrate (DE-PS 42 23 360). Even in consideration of this latest state of the art, it is important to maintain a sufficiently low atmospheric humidity during production and storage of an estradiol-TTS in order to avoid large-area precipitation of the poorly soluble estradiol-semihydrate.
To this end, a package having a low water-vapor permeability can be used in principle. However, owing to the small estradiol amounts contained in today's TTSs, very small amounts of humidity are sufficient to cause precipitation of the estradiol-semihydrate. If, for example, 2 mg of estradiol (anhydrous) are present in a TTS in dissolved form, an amount (calculated on the basis of the molecular-weight ratios) of only 66.1 .mu.g of water can cause complete precipitation. Using conventional packaging means, it is therefore very difficult to exclude entry of such small quantities of moisture over storage periods of several years.